1. Field of the Invention
The invention relates to mechanical structural repair of advanced superalloy components, and more particularly to mechanical structural repair of superalloy material turbine blade castings of the type used in gas turbines, by use of splice inserts, in a manner that does not significantly reduce mechanical structural or material properties of the repaired blade.
2. Description of the Prior Art
“Structural” repair of gas turbine superalloy components is commonly recognized as replacing damaged material with matching alloy material and achieving properties such as strength that are close to the original manufacture component specifications (e.g., seventy percent ultimate tensile strength of the original specification). In comparison, “cosmetic” repair of such alloys is recognized as replacing damaged material with unmatching alloy material of lesser structural property specifications, where the localized original structural performance is not needed, in order to restore the repaired component's original profile geometry. An example of cosmetic repair is for filling surface pits on a turbine blade airfoil in order to restore its original aerodynamic profile, where the blade's localized exterior surface is not critical for structural integrity of the entire blade. Cosmetic repair is often achieved by using oxidation resistant weld or braze alloys of lower strength than the blade body superalloy substrate, but having higher ductility and lower temperature application that does not negatively impact the superalloy substrate's material properties.
Structural repair of nickel and cobalt based superalloy material that is used to manufacture turbine components, such as cast turbine blades, is challenging, due to the metallurgic properties of the finished blade material. The finished turbine blade alloys are typically strengthened during post casting heat treatments which render them difficult to perform subsequent structural welding. For example, when a superalloy, such as CM247, is welded with rods of the same or similar alloy, the blade is susceptible to strain age cracking within proximity to the weld. Often the only solution is to scrap damaged turbine blades that require structural repair, because past experience has shown limited success of such structural repairs. Thus repairs have been limited to those that have in the past been proven to be performed successfully by cosmetic welding, employing more ductile welding rod filler materials with reduced structural strength.